The present invention relates generally to a method of and apparatus for accurately detecting and measuring the mean concentrations, over time, of thoron and/or radon in a gas mixture, such as the ambient atmosphere in a mine shaft where these substances are likely to be present. Specifically, the method and apparatus of the invention may be adapted for measuring the mean radon and thoron concentrations in test gas mixtures for exposures of more than 100 pCi liter days.
Many naturally occurring elements, most notably those of atomic numbers from 81 to 92, undergo spontaneous radioactive decay. A naturally occurring radioactive element emits alpha particles (having a discrete kinetic energy) or beta particles (which may be accompanied by gamma rays) and thereby forms a new isotope. The new isotope may itself be radioactive so that further alpha and/or beta decay occurs until a stable isotope is finally formed. Thus, most naturally occurring radioactive elements are said to undergo a radioactive decay series which includes several isotopes.
The emission of an alpha particle decreases the atomic weight of an element by four units and its atomic number by two units. When a radioactive element emits a beta particle, the atomic number of the element increases by one unit but the atomic mass remains substantially unaltered.
Radium (usually denoted .sup.226 Ra) is solid at ambient temperatures and pressures. The first stage of its radioactive decay series results in the formation of radium emanation, or radon (.sup.222 Rn), a noble gas. The decay of .sup.226 Ra to .sup.222 Rn is accompanied by the emission of an alpha particle having a discrete kinetic energy of about 4.8 MeV, a mass equivalent to a helium nucleus and a charge equivalent to +2e. .sup.222 Rn gas undergoes further radioactive decay via alpha and beta emissions, until a stable form of the element, sometimes referred to as radium G is finally formed. Radium G is isotopic with lead (often denoted .sup.206 Pb) and does not itself exhibit any further radioactivity. The radioactive decay of .sup.226 Ra thus includes a series of daughter products, only one of which, .sup.222 Rn, is gaseous. In each instance, however, the formation of a new daughter product results in the emission of an alpha or beta particle.
Thorium (which has an atomic number 90, and atomic weight of approximately 232, often denoted .sup.232 Th) also undergoes radioactive decay in a series via alpha and beta emissions, like .sup.226 Ra. Again, only one daughter product of the thorium decay series, commonly referred to as thorium emanation, or thoron (.sup.220 Tn) is gaseous. As .sup.220 Tn undergoes further radioactive decay, the final daughter product is often referred as thorium D and has an atomic number 82 and an atomic weight 208. Like radium G, thorium D is isotopic with lead and does not exhibit further radioactivity.
When any radioactive element undergoes alpha decay, the emitted alpha particle is imparted with a discrete kinetic energy. The alpha particle is propelled in a random, linear direction from the decaying atom. The alpha particle will therefore travel a given distance through the medium into which it is ejected, until it is stopped by the absorption of its kinetic energy by the mass of the medium through which it passes. Thus, for a given radioactive gas atom undergoing alpha decay in air, the alpha particle will be propelled in a linear direction from the point source of the decaying atom, and will terminate its travel at some point corresponding to a spherical surface about the decaying gas atom. This distance of travel will, of course, vary with mass of the medium and the kinetic energy of the alpha particle. This distance may be referred to as the "range" of the alpha particle and, in air, is about 0.8 to 0.9 centimeters per MeV of kinetic energy. Similar range-energy relationships exist for alpha particles passing through other materials, such as metals, plastics, etc.
Because .sup.226 Ra and .sup.232 Th are present in varying but substantial concentrations in the earth's crust, .sup.222 Rn and .sup.220 Tn gases, and the daughter products generated in their radioactive decay series, are often present in the atmosphere in varying concentrations. However, because only .sup.222 Rn and .sup.220 Tn are gaseous (under ambient conditions), the daughter products produced therefrom generally settle out of the atmosphere quite rapidly. Nonetheless, some solid daughter products may remain present in the atmosphere as aerosol particulates.
For many years it has been known that .sup.222 Rn and .sup.220 Tn may be concentrated in particular environments that tend to contain or concentrate these gases or in environments which are located particularly close to a source of the same. Thus, .sup.222 Rn and .sup.220 Tn gases may concentrate in the atmosphere of buildings and homes (particularly in basements) and in underground cavities such as and mines and geological faults.
While the concentrations of .sup.220 Tn in buildings has not been well studied, it is estimated to be at least about ten percent of the concentration of .sup.222 Rn and perhaps many times greater than that amount. Because both .sup.222 Rn and .sup.220 Tn are gaseous, they are readily absorbed through inhalation by workers or residents in environments having substantial concentrations of either isotope. Thus, such persons may be exposed to low level radioactivity by the decay of .sup.222 Rn and .sup.220 Tn over a substantial period of time. In particular, exposure to higher concentrations of radioactivity may result from the accumulation of the nongaseous but radioactive daughters of these gaseous isotopes. Thus, accurate measurement of .sup.222 Rn and .sup.220 Tn in homes and working environments is of particular importance because low levels of radioactivity have been suspected of playing substantial roles in the initiation of cancers of the lung, etc. Furthermore, it is unknown whether the detrimental health effects from exposure to .sup.222 Rn or .sup.220 Tn differ in any substantial manner. This indicates a need for accurate, independent measurements of these substances in atmospheres where human exposure is probable.
Because of the known health hazards posed by .sup.222 Rn gas, many apparatus and methods have been developed for detecting the presence and mean concentration (over time) of this gas. Most such apparatus utilize an alpha particle sensitive material for detecting the presence of and the concentration of .sup.222 Rn gas. Alpha particle sensitive materials are well-known in the art and are defined herein as any material which permits the detection of an alpha particle collision with the material. Most frequently, a polymeric material such as CR39 is used to detect the presence of alpha particles. An alpha particle penetrating such a polymeric material causes localized damage to the material in a conically shaped region surrounding the linear path of the particle. Because the alpha decay of .sup.222 Rn and .sup.220 Tn and each of their daughter products is associated with a discrete kinetic energy imparted to the alpha particle, the depth of penetration of the alpha particle in the polymeric material can determine which isotope or daughter product has decayed.
The alpha particle sensitive materials may be etched by exposure to a hot alkaline solution (e.g., 6M NaOH or KOH at 60.degree. C., a process hereinafter referred to as "developing"). The radiation damaged zones created by the alpha particle penetration (also referred to herein as "alpha tracks") will etch at a rate faster than the balance of the material and will therefore reveal an etch pit at the site of each alpha particle penetration. The depth of the pit, as measured from the surface of the alpha particle sensitive material, is directly proportional to the kinetic energy of the alpha particle at the time of striking the material surface, as well as the angle at which the alpha particle entered the surface of the material.
It is understood that radioactive decay which involves the emission of beta particles does not affect the ability to detect alpha particle penetration in alpha particle sensitive materials. It is also understood in the art that the localized damage imparted to the alpha particle sensitive material is greatest in the region adjacent the terminal point of travel of the alpha particle. Thus, detection of the alpha particle collision is easiest at a zone adjacent the maximum depth of penetration.
.sup.222 Rn, .sup.220 Tn and their daughter products emit alpha particles having discrete kinetic energies, which kinetic energies fall within given ranges. Because the ranges overlap, it has heretofore been difficult to accurately detect the mean concentration of .sup.220 Tn in the presence of .sup.222 Rn. Thus, simple exposure of an alpha particle sensitive material to an atmosphere suspected of containing one of these gases, or a gas mixture which includes both .sup.222 Rn and .sup.220 Tn, will reveal alpha tracks of like or similar depths, generated by both radioactive isotopes and their daughter products.
For this reason, prior art apparatus have been constructed to include a housing having a chamber formed therein and an alpha particle sensitive material contained in the chamber. The housing is impenetrable to alpha particle radiation from either .sup.222 Rn, .sup.220 Tn, their daughter products or other stray sources of alpha radiation ("noise"). The housing has an opening formed therein which communicates with the chamber and which is covered with a semipermeable membrane that permits only the passage of .sup.222 Rn gas therethrough, excluding .sup.220 Tn from the chamber. In this manner, the alpha tracks created in the alpha particle sensitive material are a result only of the radioactive decay of .sup.222 Rn and its daughter products.
Thus, in order to determine the mean concentrations of both .sup.222 Rn and .sup.220 Tn over long periods of time, two detectors have heretofore been required: a first detector which determines the concentration of both .sup.222 Rn and .sup.220 Tn; and a second detector which determines the concentration of .sup.222 Rn only. The mean concentration of .sup.220 Tn in the test atmosphere has then been estimated by the differential count in alpha tracks as registered in the alpha particle sensitive material of the two detectors.
This method, however, has several drawbacks. The efficiency of the .sup.222 Rn only detection device (including the semipermeable membrane) has been reported to be only about 70% of that of the .sup.222 Rn plus .sup.220 Tn device. Furthermore, because two detectors are employed, each of which has a different efficiency, and each of which may include an alpha particle sensitive material from different batches of manufacture, further discrepancies in measuring accuracy may be expected. Finally, because two separate detectors are used, their locations may be remote enough to even further degrade the accuracy of the measurement. Thus, in the prior art measurement of the concentration of .sup.222 Rn and .sup.220 Tn in a test atmosphere, the compounding of measurement error may be expected.
Therefore, there exists a need for a novel method and apparatus capable of accurately detecting the presence and mean concentration of .sup.220 Tn in a test gas mixture, which test gas mixture may include radon. Furthermore, there exists a need for a novel method and device which can simultaneously and accurately detect the presence and relative concentrations (over time) of both .sup.222 Rn and .sup.220 Tn in a test gas mixture, such as air, which includes both .sup.222 Rn and .sup.220 Tn.